Photolithographic technology for optically transferring circuit patterns to semiconductor wafers is vital to the integration of LSI chips. The aligners used in photolithography are primarily reduction and projection exposure systems known as steppers. That is, the light that passes through a reticle pattern irradiated with a source of illumination is projected by means of reduction and exposure optics onto a photosensitive substance present on a semiconductor substrate, thereby forming a circuit pattern. The resolution of the projected image is limited by the wavelength of the light source used. Accordingly, the desire for even finer pattern linewidths has led to the use of light sources of increasingly shorter wavelengths in the ultraviolet region.
Lately, KrF excimer lasers (wavelength, 248 nm) and ArF excimer lasers (wavelength, 193 nm) are being used as light sources to generate light in the deep ultraviolet region (DUV light), and F2 lasers (wavelength, 157 nm) are being used as light sources to generate light in the vacuum ultraviolet region (VUV light).
Efforts are currently underway to use EUV light sources (wavelength, 13.5 nm) which output light in the extreme ultraviolet region (EUV light) in order to carry out fabrication to an even smaller pattern linewidth.
One type of system for generating EUV light is the laser produced plasma (LPP) system. In a LPP-based EUV light source, a short-pulse laser beam is directed at a target, exciting the target to a plasma state and causing EUV light to be generated. This light is then collected with a collection lens, and the EUV light is output to the exterior.
FIG. 11 is a schematic showing the configuration of an EUV light generator which can be used as the light source for an aligner.
A collector mirror 41 which collects EUV light is provided at the interior of the chamber 4. The EUV light collected by the collector mirror 41 is sent to illumination optics (not shown) outside of the chamber 4, where the EUV light is used to form a semiconductor circuit pattern on a semiconductor wafer.
The interior of the chamber 4 is evacuated using a vacuum pump or other suitable means, thereby placing it in a vacuum state. This is done because EUV light has a short wavelength of 13.5 nm and will not propagate efficiently except in a vacuum.
A liquid target 1 which is an EUV light generating substance is released as a droplet from the nozzle 40. The target 1 may be, for example, liquid xenon (Xe). The target 1 has a diameter of about 10 μm.
The laser oscillator 10, which may be a YAG laser, for example, pulse generates a near-infrared laser beam L which irradiates a target 1. The direction of irradiation by the laser beam L is perpendicular to the direction in which the target 1 travels.
When the laser beam L irradiates the target 1, the target 1 is excited to a plasma 2 state and generates EUV light. The plasma that forms has a diameter in a range of about several tens of microns to 1 mm. The generated EUV light spreads out in all directions from the plasma 2 as the center. Because the collector mirror 41 is arranged so as to encircle the plasma 2, the EUV light which spreads out in all direction is collected by the collector mirror 41, which reflects the collected EUV light and sends it to illumination optics.
A portion of the target 1 fragments and scatters under the influence of shock waves during plasma generation, forming debris 3. The debris 3 includes high-speed ions and residues that were not transformed to plasma.
A collecting tube 130 is provided in the travel path of the target 1 for the purpose of collecting unconsumed residues from targets 1, and any targets 1 not irradiated by the pulsed laser beam L. The collecting tube 130 is equipped with a collecting mechanism 131. The collecting mechanism 131 includes a filter and a vacuum pump, and functions to trap debris 3 collected within the collecting tube 130 and discharge it to the exterior by applying a vacuum.
However, most of the debris 3 is not collected by the collecting tube 130 and remains suspended within the chamber 4. Leaving the debris 3 within the chamber 4 is desirable neither for the durability of the EUV light generator nor for the light output efficiency.
That is, high-speed ion debris 3 collides with optical elements such as the collector mirror 41, forming marks on smooth reflecting surfaces such as those of the collector mirror 41 and shortening the life of the optics.
Also, when the debris 3 deposits on an optical element such as a collector mirror 41, this lowers the reflectivity of EUV light, in turn lowering the output of EUV light.
Moreover, gasification of the debris 3 lowers the degree of vacuum within the chamber 4, reducing the propagation efficiency of the EUV light and decreasing the output of EUV light.
The following conventional art exists for collecting debris.
Conventional Art 1
Patent Document 1 below describes a technique for discharging debris from a “cryotarget” (a substance which is a gas at ambient temperature) outside of the chamber using a vacuum pump.
Conventional Art 2
Patent Document 2 below describes a technique in which a transmission filter is placed on the incident side of a return mirror in an illumination optical system, and debris included in the path of EUV light reflected by the collector mirror is adsorbed or absorbed with the transmission filter.
Patent Document 1: Japanese Patent Application Publication No. 2002-289397
Patent Document 2: Japanese Patent Application Publication No. 2000-349009